In a number of different fields such as telecommunications, scientific instrumentation, optics and so forth, there is a need for more complex optical thin film coatings to meet the requirements of advanced applications. In the past, due to limitations in design techniques, deposition equipment and thickness monitoring instrumentation, it has not always been possible to achieve the desired filter specifications. However, recent advances in all three of these areas have allowed more complex coatings to be designed and fabricated.
For example, in the area of optical thin film design there have been some dramatic improvements over the past few years in the ability to design coatings without any starting design for a given set of materials. Indeed, state of the art of thin film design has advanced to the point where very complex coatings can be found that meet all but the most stringent filter specifications.
The second advance is in energetic deposition methods, which have dramatically changed the field of thin film manufacture over the past twenty years. Previously, and still true to a large extent, most optical coatings have been fabricated using e-beam evaporation or thermal evaporation. However, materials deposited by these processes are generally quite porous, resulting in filters with poor humidity and temperature stability unless they are specially protected. Newer energetic deposition processes, such as ion-assisted evaporation, reactive ion-plating and magnetron sputtering give rise to films that have bulk-like properties which results in filters with good-to-excellent temperature and humidity stability. More importantly perhaps, materials deposited by these energetic techniques have optical constants that are very reproducible from run-to-run. With bulk-like materials and optical constant reproducibility, it becomes more feasible to manufacture complex optical coatings on a routine basis.
The third advance has been in methods used to control or monitor the film thickness during deposition. In particular, optical monitoring techniques have greatly improved over the past ten years. Sophisticated and inexpensive wideband monitors for the visible region are now readily available and infrared photodiode arrays are becoming more common. With these wideband monitors, it is now possible to more accurately determine the thickness of a deposited layer.
In the past, for limited filter quantities, it may not have been economical to design and fabricate small quantities of complex thin film filters even though there is a large market for such custom or prototype coatings. Part of the reason for this is the time it takes to design a coating and the number of trial deposition runs that usually have to be made before a filter is successfully fabricated. If, in addition, the deposition system requires constant operator intervention to ensure the coating is accurately deposited, the cost will be further increased.
In 1991 a project was begun at the National Research Council of Canada (NRCC) to develop an Automated Deposition System (ADS) that could routinely fabricate complex optical coatings automatically without the need for operator intervention during the deposition process.
The original rf-ADS at the NRCC consisted of a cryo-pumped chamber having a rotatable substrate; three rf-sputtering targets; and a wideband optical monitor that will be described in more detail below. The targets and chamber were designed and built by Corona Vacuum Coaters. In the ADS, the sputtering targets and substrates are mounted vertically. The targets are usually metal or semiconductor, so that the system uses rf reactive sputtering for dielectric layers. Typical deposition rates for materials like Nb.sub.2 O.sub.5 and SiO.sub.2 using rf sputtering are .about.0.1 nm/s for a target-to-substrate distance of .about.12 cm, a total pressure of .about.3 mTorr and an oxygen-to-argon flow ratio of .about.1.0. The substrate is controlled by a stepper motor that can be used to swing the substrate to the various target positions. The deposition system was controlled by a Techware Systems (known as Brooks Automation (Canada)) PAL68000 controller. An operator can automatically initiate a number of different sequences including pumping down the chamber, starting a deposition run or venting the chamber.
A real-time process control algorithm accurately controls the film deposition thickness for low rate deposition, i.e. deposition rates of the order of 0.1 nm/s. This technique requires a wideband optical monitor that is able to make accurate, absolute, transmittance measurements over a sufficiently wide spectral range. Also, because it is difficult to continuously monitor the deposition of a layer in sputter systems where the target-to-substrate distance is small, this method relies on making one or two transmittance measurements near the end of a layer deposition.
The optical monitor consists of a quartz-halogen lamp source, light delivery optics, and a wideband detector. The collimated light from the source passes through the chamber and is collected by an achromatic lens that focuses the light through a shutter onto the circular aperture of a fiber-optic bundle. At the other end of the bundle, the fibers are arranged to form a slit at the entrance to a monochromator. The light is then dispersed onto a 512-element Hamamatsu photodiode array. The grating was chosen such that optical monitor could measure over a 380 to 860 nm spectral range. In order to make absolute transmittance measurements, it is possible to rotate the substrate in and out of the optical monitor light path. This allows intensity measurements to be made with and without the substrate. These measurements, after subtracting the background, are then normalized to provide the absolute transmittance of the substrate. This measurement process is completely automated.
The last key element in the ADS system is an integrated thin film program that can be used to first design complex multilayer coatings, based on the optical constants of the materials deposited by the ADS, and then can be subsequently used to oversee the manufacture of the coating. This program can determine the current or previous layer thicknesses from the absolute transmittance measurements of the optical monitor. In addition, the program can reoptimize the remaining layers in the multilayer system at any time during deposition in order to achieve the desired filter specifications.
The program is integrated with the deposition controller in such a way that it does not need to know any details concerning the actual deposition system. When it requires a particular layer to be deposited, it is sufficient to pass down the layer material, the desired thickness and a process name. The controller software then interprets this information to determine the target that the substrate should be rotated to; the length of time the substrate should remain in front of the target; and the deposition parameters that should be used during the deposition. By separating the thin film control algorithm and the deposition system in this way, it is possible to completely change the deposition system and processes without affecting the thin film program.
For a given layer during deposition, the thickness process control algorithm essentially has three stages:
I. termination of layer deposition, PA1 II. determination of layer thickness deposited, and PA1 III. reoptimization of remaining layer thicknesses. PA1 a deposition chamber comprising at least two sources of materials to be deposited by reactive deposition; PA1 means in said chamber for measuring an optical property of the deposited layers; PA1 means for fitting theoretical values derived from a model of the deposited layers to the corresponding actual values obtained from the measurement of said optical property; and PA1 means for continually controlling a process variable to ensure homogeneity of the deposited layers so that a valid thickness determination can be made from said theoretical model.
With the ADS, stages I and II are combined together. The first stage, concerned with the termination of a layer deposition, can be based on time alone since sputter deposition is being used in this system. Since the uncertainties in the deposition are typically of the order of 1-3%, for a reasonably well-controlled process, this implies that in order not to overshoot the desired layer thickness, the target thickness first specified should be around 95-97% of the desired thickness. Once this sub-layer has been deposited it is then necessary to determine the layer thickness actually deposited, i.e., stage II. As described above, this is achieved by performing a wide-band optical monitor measurement directly on the substrate of interest, or on a witness slide. The thin film program then uses this information to determine the current layer thickness deposited. If the layer thickness is not within the specified thickness tolerance with respect to the desired thickness, stages I and II can be repeated. Since the remaining layer thickness is typically quite small, i.e., less than 5 nm or so, any uncertainties in the deposition rate are not very important for the second sub-layer. The determined layer thickness is also sent back to the controller which uses this information to update the current deposition rates in order to minimize the number of sub-layers required. The final stage concerns the reoptimization of the remaining layer thickness once the layer has been finished. For some filters, this stage is not necessary, however for other filters this reoptimization is critical if the desired filter specifications are to be met.
Crucial to fabricating a filter with a good performance in the ADS is the assumption that the deposited filter can be accurately modeled. This then allows an accurate determination of the layer thickness based on the experimental transmittance data. If this is not achieved, then it will not be possible to accurately control the fabrication of the multilayer. Fortunately, once the optical constants of the materials have been well characterized, any problems with the layer determination can usually be handled by modifying the layer determination process or by adjusting the multilayer solution. Using the thin film program, it is possible to first simulate the deposition process in the ADS before making a coating and to see if the layer determination strategy is acceptable.
In setting up the layer determination process, a number of factors need to be considered that depend on the nature of the filter that is being fabricated. For instance, a minimum layer thickness is required so that there is a detectable change in the transmittance of the filter after it has been deposited. If a multilayer has been fabricated and the resulting filter performance is not within specifications, it is possible to enter a replay mode since all the transmittance measurements made during a deposition run are saved. With this feature, an operator can quickly review the multilayer deposition and pin-point where the problem layers were. It is then possible to re-adjust some layer determination parameters in the replay mode to see if a better solution can be obtained. For example, an accurate determination of the thickness of a given layer may be hard to achieve depending on the multilayer system. In this case, it may be best to deposit that current layer by time alone and use transmittance measurements after the next layer has been deposited to accurately determine the thickness of both layers. Reoptimization of the remaining layers can then be used to take into account any thickness errors in these layers.
Many different types of filters have been fabricated on the rf-ADS over the past several years. These include all-dielectric coatings such as edge filters, narrowband transmittance filters, notch filters, colorimetric filters, antireflection filters, custom bandpass filters as well as metal/dielectric coatings.
The rf-ADS has proved capable of fabricating complex optical filters automatically. However, there are some limitations to this system that preclude its use as a production deposition system; namely, the low deposition rates and the somewhat limited thickness uniformity. Some filter designs can take up to 21 hours to deposit. Hence, it would be very beneficial to be able to greatly increase the deposition rates while at the same time also increasing the thickness uniformity.
High rate sputter sources capable of depositing films at rates of the order of 0.7 nm/s have recently become available. Unfortunately, when such sources were employed, it was found that the film thicknesses could not be determined with the required degree of accuracy. As a result, the ADS system, which works well at low rates, could not be used to deposit complex films automatically at commercially viable rates.
An object of the invention is to provide an automated deposition system capable of operating at relatively high rates.